Screening systems are widely used in manufacturing processes to separate larger particles from a liquid/solid suspension. For example, applications for screening systems include processing chemical pulp, mechanical groundwood, bleached or unbleached kraft, old corrugated containers, mixed news, drink stock, waste paper or any other type of fiber.
The system typically receives several thousand gallons per minute of liquid/solid suspension, and separates the larger particles using a pressure screen which typically provides an accept flow (smaller particles) and a reject flow (larger particles). An example of a pressure screen is the Model 400 available from Voith Sulzer, the assignee of the present invention.
The system typically includes an automated controller which controls the overall system operation, monitors the system for component failures/flow anomalies, and automatically initiates corrective action. To detect an obstruction across the pressure screen (typically caused by accumulated particles which are too big to pass through the screen), the controller monitors pressure across the screen. If the pressure exceeds a fixed, predetermined differential pressure threshold value, the controller initiates the necessary corrective action in order to remove the blockage. In many prior art systems, the controller simply sounds an alarm when the pressure across the screen exceeds the threshold value, and system operating personnel manually take the necessary corrective action. Significantly, in both automatic and manual systems, initiating corrective action involves interfering with the conventional flow processing, and therefore reduces the processing efficiency of the system.
The predetermined differential pressure threshold value is often selected during the system design of the screening system and remains fixed. Selecting this threshold value has involved a balancing of several factors. If the value is set too low, the controller initiates unnecessary corrective action. In contrast, if the value is set too high, an excessive amount of the liquid/solid suspension will be regularly routed to the reject flow.
A problem with both automatic and manual systems is that the threshold value remains fixed. Specifically, what might be a "high" differential pressure for one accept flow rate or system operating condition, might well be "normal" for another accept flow rate or operating condition.
Therefore, there is a need for an improved system and method for monitoring a pressure screen for flow disturbances.